Abstract - In a systemic approach, the breeder can be considered as the decisional
component of the livestock system, whereas animals are usually depicted to
be part of its biotechnical component. The animal itself is a biological
system whose ability to survive, grow, reproduce and cope with the
environnement and livestock practices play a major role in the ability of
the livestock system to sustain. In such a conceptual representation of the
system, the reproductive females draw a peculiar attention since they
determine in a great part the productivity and the durability of the system
through their abilities to maintain their own production level (milk
production, numeric productivity) and to save their reproductive efficiency
(repeated pregnancies and lactations) over years. Considering the animal
level and its lifespan, it is clear that the abilities to adapt rely on
behavioural and physiological regulatory processes. The study of the
biological mechanisms involved in the adaptation to undernutrition is
particularly interesting since regulatory processes implied in energy
metabolism may interfere directly or indirectly with the reproductive
function, and consequently, with the durability of the livestock system. A
biological significance of such relationships between nutrition and
reproduction is given that they allow the female to be informed about the
associated risk of entering a productive process facing the uncertainty of
the nutritional context. Although the general mechanisms implied in the
ability to adapt to the underfeeding constraint are conserved among
ruminants, the thresholds (or priorities) may largely differ according to
the breed within the same species. Hence, in order to evaluate the ability
of the ruminant livestock systems to perpetuate in hard environments
(maintaining their production levels) or to assess sustainable objectives
(opening bushy landscapes by increasing grazing pressure), animals' inherent
adaptive potentialities have to be well known.